In the semiconductor industry, wire bonding is commonly performed to electrically interconnect an integrated circuit (IC), such as a semiconductor die or chip, to various structures, such as a metal leadframe. Wedge bonding, for example, is a conventional method used to bond thin wires, such as thin aluminum or gold wire, between a bonding point on the semiconductor die to another point, such as a lead finger of the leadframe. Conventionally, in order to initiate a bond, the wire is pressed against the IC and/or leadframe with a tip of a bonding tool. The bonding tool is ultrasonically vibrated for a period of tens of milliseconds, wherein a plane of motion of the tip of the bonding tool is generally parallel to the surface of the semiconductor chip to which the bond is to be formed. The combination of a static load of the bonding tool normal to the chip's surface against the wire and chip coupled with the vibration at the tip of the tool causes the wire to plastically deform, thus simultaneously joining the bond wire with atoms of the material composing the chip's or leadframe's surface, and accordingly, providing a cold weld between the wire and the chip or leadframe.
One problem associated with wire bonding is the handling of the wire throughout the bonding process, and the breaking or severing of the wire after the bond has been made. To perform these functions, most conventional fine wire wedge bonding tools break the wire by clamping and/or pulling the wire away from the bonded substrate. Typically, with small diameter wires (circular cross-section wires having diameters of less than 0.025 inches), such a pulling action is sufficient, and does not cause significant damage to the substrate. However, with the ever-increasing demands for speed and performance of ICs, ribbon wires have been introduced, wherein a cross-sectional area of the ribbon wires is significantly greater than previously-used round cross-section wires, thus providing greater electrical current-carrying capabilities for the wire. Accordingly, when the conventional bonding tools are used with the thicker ribbon wires, the pulling force required to break the ribbon wire tends to deleteriously deform the chip and/or leadframe. As a consequence, various configurations for bonding tools have been formulated, such as one wherein a blade is pressed against the chip and/or leadframe in order to deform or cut the ribbon wire prior to pulling the bonding tool away from the substrate.
FIGS. 1A–1C illustrate one such conventional bonding device 10 during various operations, wherein a ribbon wire 15 is bonded to a substrate 20, such as a leadframe finger or semiconductor chip by an ultrasonic bonding tool 25. As illustrated in FIG. 1A, the bonding tool 25 is pressed against the ribbon wire 15, thus compressing the ribbon wire between the bonding tool and the substrate 20. At this point, the bonding tool 25 ultrasonically vibrates, thus cold-welding the ribbon wire 15 to the substrate 20. FIG. 1B illustrates a cutter 30 that is pressed against the ribbon wire 15, wherein the cutter significantly thins or cuts through the ribbon wire by the force F exerted by the cutter. FIG. 1C illustrates the bonding tool 25 being pulled away from the substrate 20, wherein a clamp 35 further pulls the ribbon wire 15 from the substrate, thus fully severing the ribbon wire generally at a tip 40 of the cutter 30.
One problem with the conventional bonding of ribbon wires, however, is that the force F of the cutter 30 against the ribbon wire 15 illustrated in FIG. 1B typically translates into a force on the substrate 20, wherein the substrate is potentially permanently deforming and/or damaged by the cutting operation. For example, as in the case of the ribbon wire 15 being bonded to a thin leadframe finger, the force F significantly deforms the leadframe finger (as illustrated by arrow 45), wherein the deformation remains after the cutter is pulled away from the substrate 20. Furthermore, if the ribbon wire 15 is not completely severed, the pulling force may further bend the leadframe finger in a direction opposite of the arrow 45 when the clamp 35 pulls the ribbon wire away. Alternatively, as in the case of the ribbon wire 15 being bonded to a chip, the cutting force F can deleteriously impact the chip, such as potentially damaging metallization layers or other layers of the IC.
Therefore, a need currently exists for a reliable process and apparatus for bonding ribbon wires to substrates, wherein damage to the substrate is substantially minimized.